This invention relates to silver alloy compositions.
This invention has particular reference to sterling silver alloy compositions of silver content of at least 92.5% for jewelry, flatware, coinage and other applications where a work hardening alloy is required and for illustrative purposes reference will be made to this application. However, it is to be understood that this invention could be used to produce other types of silver alloys suitable for use as for example, electrical contacts or the like.
In general, silver as a material for the production of silver jewelry, certain coinage and the like is specified to be sterling silver comprising at least 925 parts per thousand by weight fine silver and is specified as xe2x80x9c0.925 silverxe2x80x9d. 0.925 silver accordingly typically comprises an alloy 92.5% by weight silver, generally alloyed with copper for hardness traces of other metals as additives or impurities.
Conventional silver alloys of the 0.925 type have several disadvantages in a manufacturing jewelry and other materials engineering contexts. Principal limitations include a characteristic firescale formation tendency attributable to oxidation of copper and other metals at the surface of cast or hot worked pieces. Additionally, traditional alloys have exhibited undesirable porosity in the recast metal and less than desirable grain size properties.
Several formulations have been proposed to overcome one or the other of the aforementioned disadvantages. U.S. Pat. Nos. 5,039,479 and 4,973,446 disclose alloys of silver and master alloys for the production of such silver alloys having superior qualities over conventional alloys, and including, in addition to silver, controlled amounts of copper and zinc, together with tin, indium, boron and silicon.
The compositions exhibit reduced porosity, grain size and fire scale production, and have acquired wide utilization in silver jewelry production. It is presumed but not established that the addition of zinc to such compositions provides at least a degree of antioxidant properties to the compositions when hot worked and improves colour, thus limiting the formation of principally copper oxide based fire scale, and reducing silver and copper oxide formation resulting in formation of pores in the cast or recast alloys. Silicon appears also to function as an antioxidant, thereby reducing firescale formation.
A disadvantage of the hereinbefore described firescale resisting alloys is that the alloys exhibit poor work hardening qualities thus not achieving the mechanical strength of traditional worked 0.925 silver goods.
The present invention aims to provide silver alloy compositions which substantially alleviate at least one of the foregoing disadvantages. A further object of the present invention is to provide silver alloys having the desirable properties of reduced fire scale, reduced porosity and oxide formation and reduced grain size relative to traditional sterling silver alloys whilst providing improved work hardening performance over the current firescale resistant alloys. Other objects and advantages of this invention will hereinafter become apparent.
With the foregoing and other objects in view, this invention in one aspect resides broadly in firescale resistant, work hardenable jewelry silver alloy compositions comprising:
0.5-6% by weight copper;
0.02-7% by weight of a firescale resisting additive selected from one or a mixture of zinc and silicon, and
0.01-2.5% by weight germanium.
The silver content of the alloy may be selected to be in the amounts commonly specified for grading silver. For example, the alloy may comprise from about 89 to 95% by weight silver. Preferably, the alloy contains a proportion of silver required for the graded application to which the alloy is to be put, such as 0.925 silver, that is at least 92.5% by weight, for sterling silver applications and at least 90% by weight for coinage.
The copper content of the alloy may be selected according to the hardness required of the cast alloy. For example, for manufacturing jewelers, 0.925 alloy, the copper content may advantageously be in the range of from about 2.0 to 3.0% by weight.
The zinc content of the alloy has a bearing on the colour of the alloy as well as functioning as a reducing agent for silver and copper oxides. Preferably, the amount of zinc used is selected to be between about 2.0 and 4.0% by weight. The silicon content of the alloy is preferably adjusted relative to the proportion of zinc used to provide the desired firescale resistance whilst maintaining a suitable colour commensurate with the zinc content of the alloy, and may for example advantageously fall within the range of about 0.15 to 0.2% by weight.
The germanium content of the alloy has surprisingly resulted in alloys having work hardening characteristics of a kind with those exhibited by conventional 0.925 silver alloys, together with the firescale resistance of the hereinbefore described firescale resistant alloys. In general, it has been determined that amounts of germanium in the alloy of from about 0.04 to 2.0% by weight provide modified work hardening properties relative to alloys of the firescale resistant kind not including germanium. However, it is noted that the hardening performance is not linear with increasing germanium nor is the hardening linear with degree of work.
Preferably, the alloy also includes rheology modifying and other additives to aid in improving the castability and/or wetting performance of the molten alloy. For example, about 0.0 to 3.5% by weight of a modifying additive selected from one or a mixture of indium and boron may be advantageously added to the alloy to provide grain refinement and/or reduce surface tension, thereby providing greater wettability of the molten alloy. Where used, preferably the amount of boron utilized in the composition is from about 0 to 2% by weight boron and/or about 0 to 1.5% by weight indium. Other alloying elements may be added such as gold, tin or platinum. Where tin is included in the composition, this may be advantageously used up to about 6% by weight, and is preferably utilized in an amount of from about 0.25 to 6%.
Accordingly, in a further aspect, this invention resides in silver alloy compositions including:
81-99.409% by weight silver;
0.5-6% by weight copper;
0.05-5% by weight zinc;
0.02-2% by weight silicon;
0.001-2% by weight boron;
0.01-1.5% by weight indium, and
0.01-2.5% by weight germanium.
In a further aspect, this invention resides in silver alloy compositions including:
75-99.159% by weight silver;
0.5-6% by weight copper;
0.05-5% by weight zinc;
0.02-2% by weight silicon;
0.001-2% by weight boron;
0.01-1.5% by weight indium;
0.01-2.5% by weight germanium, and
0.25-6.0% by weight tin.
Of course, it is of advantage to the manufacturing metallurgist to be able to alloy fine silver without having to individually measure components. Accordingly, it is preferred that the compositions of the present invention be formed by the addition of a master alloy to fine silver. This also has the advantage that the master alloys are easier to transport than the made up alloys. Additionally, oxidizable components of the alloy are more stable to atmospheric oxidation when alloyed. Accordingly, in a further aspect this invention resides broadly in a method of producing firescale resistant, work hardenable silver alloy compositions and including the alloying of silver metal with a master alloy comprising, by weight:
52.5-99.85% by weight copper;
0.1-35% by weight of zinc or silicon or mixtures thereof, and
0.05-12.5% by weight germanium.
For production of the preferred modified alloys, there may be provided master alloys including additional alloying elements such as up to about 10% by weight boron, up to about 15% by weight indium and/or up to about 30% by weight tin. Accordingly, in a preferred aspect this invention resides in a method of producing firescale resistant, work hardenable silver alloy compositions including the alloying of silver metal with a master alloy comprising, by weight:
15.0-99.545% by weight copper;
0.25-25% by weight zinc;
0.1-10% by weight silicon;
0.005-10% by weight boron;
0.05-15% by weight indium, and
0.05-25% by weight germanium.
In a yet further aspect this invention resides in a method of producing firescale resistant, work hardenable silver alloy compositions including the alloying of silver metal with a master alloy comprising, by weight:
2.5-97.455% by weight copper;
0.25-25% by weight zinc;
0.1-10% by weight silicon;
0.005-10% by weight boron;
0.05-15% by weight indium;
0.05-25% by weight germanium, and
2.0-12.5% by weight tin.
In a yet further aspect this invention resides in a method of producing firescale resistant, work hardenable silver alloy compositions including the alloying of silver metal with a master alloy comprising, by weight:
2.5-97.455% by weight copper;
0.25-19.85% by weight zinc;
0.1-7.94% by weight silicon;
0.005-7.94% by weight boron;
0.05-11.92% by weight indium;
0.05-19.85% by weight germanium, and
2.0-30% by weight tin.